Make ShellCheck warning-level validation actionable

[codeforester]

Context

The canonical validation script runs ShellCheck at --severity=error, which is appropriate for keeping CI stable. A warning-level sweep currently produces recurring noise from intentional source guards, dynamic source paths, test helper wrapping, and Bats negation assertions.

This makes it hard to use warning-level ShellCheck as a meaningful regression signal.

Scope

• Decide the warning-level lint contract for production libraries, examples, validation scripts, and/or tests.
• Clean up recurring avoidable warnings where the code can stay readable.
• Add focused inline disables only where the warning is intentional and documented.
• Consider adding a separate warning-level lint command or script once the signal is clean enough.
• Keep the existing ./tests/validate.sh entrypoint stable unless the stricter lint is ready for CI.

Acceptance Criteria

• A documented warning-level ShellCheck command has low noise and clear exceptions.
• Avoidable Bats/test-helper warning patterns are cleaned up or explicitly disabled.
• Existing error-level validation remains green.
• ./tests/validate.sh passes.